This application is a national stage of PCT/EP97/03005, filed Jun. 10, 1997.
The present invention relates to a method of producing steel in an electric furnace by charging with molten pig iron.
A high proportion of steel scrap is recycled using electric furnaces such as arc furnaces. These furnaces make it possible to melt and re-use the steel scrap treated in this way in order to produce new steel products.
Some of the residual elements contained in the steel scrap such as copper, nickel, etc., cannot be separated from the steel and hence occur in the finished products. This means that the more the steel scrap is subjected to recycle operations, the greater the concentration of these residual elements. These elements cause problems for the production of certain products such as sheet steel, etc.
One way of reducing the concentration of residual elements in the steel obtained from steel scrap and of simultaneously improving the energy efficiency of the electric furnace consists in adding molten pig iron to the electric furnace. Now, because of the fairly high carbon and silicon content of the molten pig iron (typically 4.5% C and 0.6% Si), charging with molten pig iron leads to a considerable increase in the concentration of these elements in the metal bath. The result of this is a longer refining stage in the metal bath in order to reduce the carbon and silicon concentrations in the metal bath to target values, which are generally very low, e.g. for the carbon concentration the target value is between 0.05% and 0.1%.
To achieve this, traditional charging methods, after the charging with pig iron, inject a refining gas, oxygen for example, in order to reduce the carbon and silicon concentrations. With the concentrations of these elements being quite high, the rate at which oxygen is supplied must be moderated in order to avoid the disiliconising and decarburising reactions occurring too violently. In fact, in the presence of high carbon and silicon concentrations, the injected oxygen reacts very violently at the point of impact in the metal bath, leading locally to a very abrupt release of energy and of the reaction gas, such as CO for example. It is obvious that such a violent reaction is accompanied by splashes of steel and pig iron which risk fouling and damaging the cooling panels lining the inside of the furnace. Hence the need to reduce the rate at which oxygen is supplied in order to moderate the dynamics of the refining reaction.
However, due to the limited supply rate of oxygen during the refining, the latter operation takes quite a long time and, above a certain quantity of charged molten pig iron, it forms the limiting factor on the duration of a melting cycle in the furnace. In order to improve the performance of the arc furnace a regards its productivity, i.e. To reduce the duration of a melting cycle, it is therefore essential to reduce the duration of the refining in the metal bath.
The document EP-A-0 630 977 describes a process for the treatment of molten pig iron in a converter equipped with at least one electrode. It relates to a process in which the total amount of pig iron is charged into the converter before the electric arc is activated.
The object of the present invention is to propose a method of producing steel in an electric furnace by charging with molten pig iron which enables the duration of a melting cycle to be reduced.
In conformity with the invention, this objective is achieved by a method for producing steel in an electric furnace, in which a quantiity  quantity of scrap is charged into the electric furnace and molten by use of an electric arc a predetermined quantity of molten pig iron is charged into the electric furnace, after a part of the scrap is molten, and a refining gas is injected into the furnace after the planned quantity of pig iron is charged until a target value of the concentration of carbon and/or silicon in the metal bath is reached.
The quantity of molten pig iron is charged continuously and at a controlled rate without interruption of the heating by the electric arc, and injection of the refining gas into the furnace starts during the continuous charging before the concentration of carbon and/or of silicon in the metal bath has reached a predetermined limiting value, the injection taking place continuously until the end of the charging, and  the target value of the concentration of carbon and/or silicon in the metal bath is reached.
This method has the advantage, firstly, that the charging is carried out without switching off the power supply, i.e. without interruption of the heating by the electric arc. Consequently, the melting of the steel scrap is not interrupted and is carried out more rapidly than in traditional methods of charging with molten pig iron. Secondly, the refining by injection of a gas begins before the end of the charging, i.e. at a time which is earlier than in traditional charging methods. As a result of this, the duration of a melting cycle is reduced, even though the rate of injection of the gas is not increased.
Since the refining begins before the end of the charging, this method also make it possible to reduce the maximum carbon and/or silicon concentration in the metal bath during a melting cycle by an adjustment of the rates of charging and of gas injection. At the beginning of the refining, the concentration, for example of carbon, in the metal bath is in fact significantly lower than that obtained in traditional methods in which the refining begins only after charging with the total quantity of molten pig iron (it is the same for the silicon concentration). Moreover, at least a part of the carbon in the bath is oxidised as it is supplied, so that the increase in the carbon concentration in the metal bath as charging proceeds is substantially reduced and so that its concentration does not exceed a predetermined limiting value, which for the carbon concentration for example is less than 2%, and preferably less than 1.5%. The silicon concentration behaves in the same way but on a reduced scale. The predetermined limiting value for the silicon concentration is less than 0.3% for example, preferably less than 0.2%.
The carbon and silicon concentrations being limited in this way, it i possible to increase the rate of oxygen supply without the refining reaction taking place too violently. In effect, because of the limited local amount of silicon and carbon, the refining reaction is no longer localised at the point of impact of the gas in the bath but the oxygen is carried intermediately on the iron. After stirring the phases that are present (metal and slag), the iron oxide produced in this way reacts subsequently with the silicon and the carbon that it encounters at places other than the point of injection. The release of the reaction gas, such as CO for example, and the splashes therefore occur more uniformly over the whole surface of the metal bath and consequently much less violently. Thus, an increase in the rate of oxygen supply and hence in the speed of refining can be achieved without causing splashes of steel and pig iron which are too large and which risk fouling and damaging the coupling panels lining the inside of the furnace. The melting cycles of the furnace are thus shortened and the productivity of the furnace increases.
It should be noted that the charging with pig iron is achieved without stoppage of the heating by the electric arc and that the roof of the furnace remains closed for the whole duration of the charging. The latter is carried out preferably through a lateral opening in the furnace. Since the roof is closed during the whole melting cycle, inputs of air into the furnace chamber are avoided and the nitrogen input is considerably reduced. Moreover, the earlier and continuous refining leads to a continual washing of the metal bath by the reaction gases like CO. Through this washing by CO, the nitrogen dissolved in the metal bath is dissolved in the CO bubbles, which take it out of the metal bath. The nitrogen is then removed from the vessel together with the reaction gas by the furnace exhaust system. Such continuous washing thus leads to very low nitrogen concentrations in the steel produced.
As a result of this, the method according to the invention is perfectly adapted to the production of quality steels, particularly to that of very ductile steels, for which very low nitrogen concentrations are required.
The rate of supply of the refining gas and the rate of charging with pig iron are preferably adjusted so that the carbon and/or silicon concentration in the metal bath no longer increases after the refining has started. It is possible, for example, to adapt the rate of charging with pig iron to the maximum rate of oxygen supply so as to oxidise all the carbon in the bath as it is supplied. In this way, the carbon and silicon concentrations in the metal bath may be very precisely controlled during a melting cycle and it is possible to limit the maximum concentration to very low values, e.g. for the carbon, to a concentration of 0.5%
According to a preferred execution of the method, the refining gas is injected into one of the two quadrants of the furnace, which are opposite the feed opening relating to an electrode of the electric furnace. In this case, the direction of the gas injection is adjusted so that a first vertical plane containing the direction of charging and a second vertical plane containing the direction of injection intersect each other substantially in the region of the furnace electrode.
The reaction gases, such as CO for example, which are released continuously during the refining, are more abundant in the region where the fluxes of gas and pig iron meet each other than in the neighbouring regions. On leaving the metal bath, these gases displace the nitrogen in the vessel and create above the surface of the metal bath a protective atmosphere against the input of nitrogen into the bath.
Because of the very high temperatures in the neighbourhood of the electric arc, the presence of nitrogen in this region leads to a preferential nitriding of the metal bath. It is therefore greatly preferable to direct the fluxes of pig iron and refining gas so that they meet each other in the region located below the electric arc. The protective atmosphere created in the neighbourhood of the arc is consequently particularly dense and an input of nitrogen into the bath may be very effectively prevented.
It should be noted that the charging with molten pig iron can be carried out with an amount lying between 20% and 60% of the total furnace charge and that the rate of charging with pig iron is preferably less than 4% of the capacity of the furnace per minute. The rate at which oxygen is injected per tonne of furnace capacity lies with advantage between 0.5 and 1 m3 O2 per minute.
In what follows, a way of carrying out the method is compared with a traditional charging method, using an example illustrated by FIGS. 1 and 2. These show: